In this work, monomers of carvacrol (5-isopropyl-2-methylphenol), a natural monoterpene exhibiting wide range bioactivity, were trapped in a cryogenic argon matrix and characterized by infrared spectroscopy, while quantum chemical calculations at the B3LYP and MP2 levels were employed to characterize the conformational landscape of the isolated molecule. Four conformers have been localized on the potential energy surface, and the factors accounting for their relative stability were analyzed. The two most stable conformers of carvacrol, differing in the relative orientation of the isopropyl group and both having the OH group pointing away from the vicinal methyl fragment, were identified in the cryomatrix for the first time. The individual spectral signatures of the two conformers were distinguished based on the change in their relative abundance induced by exposing the matrix to broadband infrared light. Matrix-isolated carvacrol was also irradiated with broadband UV light (λ > 200 nm), which resulted in the cleavage of the OH group. Recombination of the released H atom at the ortho- or para-position of the ring resulted in the formation of alkyl-substituted cyclohexadienones. These were found to undergo subsequent valence and open-ring isomerizations, leading, respectively, to the formation of a Dewar isomer and open-chain conjugated ketenes. Decarbonylation of the photoproducts was also observed for longer irradiation times. A mechanistic analysis of the observed photochemical transformations is presented.

中文翻译：

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在低温氩气基质中分离的香芹酚的构象空间、红外诱导和紫外诱导化学

在这项工作中，香芹酚单体（5-异丙基-2-甲基苯酚）是一种具有广泛生物活性的天然单萜，被捕获在低温氩基质中并通过红外光谱表征，同时采用 B3LYP 和 MP2 水平的量子化学计算表征分离分子的构象景观。四个构象异构体已定位在势能表面上，并分析了影响它们相对稳定性的因素。香芹酚的两个最稳定的构象异构体，在异丙基的相对方向上不同，并且都具有远离邻甲基片段的 OH 基团，这是首次在低温基质中被鉴定出来。根据将基质暴露于宽带红外光引起的相对丰度变化，区分两种构象异构体的单独光谱特征。基质分离的香芹酚也用宽带紫外光 (λ > 200 nm) 照射，导致 OH 基团的裂解。在环的邻位或对位处释放的 H 原子的重组导致形成烷基取代的环己二烯酮。这些被发现经历随后的价和开环异构化，分别导致形成杜瓦异构体和开链共轭烯酮。对于更长的照射时间，也观察到光产物的脱羰。介绍了观察到的光化学转化的机理分析。基质分离的香芹酚也用宽带紫外光 (λ > 200 nm) 照射，导致 OH 基团的裂解。在环的邻位或对位处释放的 H 原子的重组导致形成烷基取代的环己二烯酮。这些被发现经历随后的价和开环异构化，分别导致形成杜瓦异构体和开链共轭烯酮。对于更长的照射时间，也观察到光产物的脱羰。介绍了观察到的光化学转化的机理分析。基质分离的香芹酚也用宽带紫外光 (λ > 200 nm) 照射，导致 OH 基团的裂解。在环的邻位或对位处释放的 H 原子的重组导致形成烷基取代的环己二烯酮。这些被发现经历随后的价和开环异构化，分别导致形成杜瓦异构体和开链共轭烯酮。对于更长的照射时间，也观察到光产物的脱羰。介绍了观察到的光化学转化的机理分析。在环的邻位或对位处释放的 H 原子的重组导致形成烷基取代的环己二烯酮。这些被发现经历随后的价和开环异构化，分别导致形成杜瓦异构体和开链共轭烯酮。对于更长的照射时间，也观察到光产物的脱羰。介绍了观察到的光化学转化的机理分析。在环的邻位或对位处释放的 H 原子的重组导致形成烷基取代的环己二烯酮。这些被发现经历随后的价和开环异构化，分别导致形成杜瓦异构体和开链共轭烯酮。对于更长的照射时间，也观察到光产物的脱羰。介绍了观察到的光化学转化的机理分析。